When an expandable endovascular prosthesis or implant, such as a stent, is deployed, it is very important to position it at the precise desired location within the patient's lumen. With some prior art stent delivery systems, as soon as the covering sheath is withdrawn to expose the underlying stent, the distal end of the stent expands in a rapid and irregular way, with the risk that one or more of the struts of the stent is deformed irregularly, such as being bent backwards. The risk of such an occurrence is increased in cases where the distal end of the delivery device on which the stent is located is not in the middle of the vessel.
Moreover, in the final stages of deployment, when the sheath slides over and beyond the distal end of the stent, this will expand in a manner which is difficult to control. This lack of control makes the placement of the implant less accurate and can also lead to damage to the intima of the vessel.
These problems tend to be exacerbated in the deployment of dissection stents for treating aortic dissections. The reason is that dissection stents tend to be very pliable and therefore require careful deployment in order not to be twisted, damaged or otherwise compromised.
In order to mitigate the problems described above, it is known to restrain the ends of the implant so as to keep it in a substantially compressed form on withdrawal of the sheath. The ends are then released to complete the deployment of the device. For example, in the case of a stent or stent-graft, the ends thereof are held tightly against the deployment catheter until released by the clinician. For this purpose, there are provided release devices at both the distal and the proximal ends of the stent or stent-graft. It is known to use release wires for the release devices, which release wires tie the ends of the stent or stent-graft until release is effected.
For example, US-2006/0,142,836 discloses a delivery device in which the proximal end of the stent graft is held by a plurality of restraining wires coupled through sutures to the apices of the proximal-most hoop of the stent. If desired, the distal end of the stent could be likewise secured by a plurality of distal end restraining wires. In order to release the stent-graft, the sheath is removed, then the proximal and distal ends released, as determined by the surgeon after final alignment of the stent-graft in the patient's lumen, by manipulation of a release mechanism which loosens the restraining wires.
US-2004/0,073,289 discloses a delivery system which is provided with a series of restraining wires for holding the proximal end of the stent-graft and a distal collar for restraining the distal end of the stent-graft. The two release mechanisms are deployable separately to release the proximal and distal ends of the stent-graft as required by the particular medical procedure.
These prior art systems can mitigate the problems described above. However, they can be difficult for a surgeon to deploy by requiring the provision of different release mechanisms at the proximal or external manipulation end of the deployment device.
In the case of certain types of implant, such as dissection stents, the deployment of the stent involves particular difficulties in light of the delicate nature of the stent, that is because of its extreme flexibility. It has been known for such a stent to become twisted as a result of rotation of the delivery device during the deployment operation, caused by having to deploy different release mechanisms and at different times.
Another problem with these prior art systems is that they necessarily take up a certain volume within the delivery device, which limits the minimum achievable diameter of the delivery device.